1. Field of the Invention
This invention relates to a method of detecting noise components contained in image signals obtained by reading out a radiation image stored on a stimulable phosphor sheet.
2. Description of the Prior Art
When certain kinds of phosphors are exposed to a radiation such as X-rays, .alpha.-rays, .beta.-rays, .gamma.-rays, cathode rays or ultraviolet rays, they store a part of the energy of the radiation. Then, when the phosphor which has been exposed to the radiation is exposed to stimulating rays such as visible light, light is emitted by the phosphor in proportion to the stored energy of the radiation. A phosphor exhibiting such properties is referred to as a stimulable phosphor.
As disclosed in U.S. Pat. No. 4,258,264 and Japanese Unexamined Patent Publication No. 56(1981)-11395, it has been proposed to use a stimulable phosphor in a radiation image recording and reproducing system. Specifically, a sheet provided with a layer of the stimulable phosphor (hereinafter referred to as a stimulable phosphor sheet) is first exposed to a radiation passing through an object such as the human body to have a radiation image of the object stored thereon, and is then exposed to stimulating rays which cause the stimulable phosphor sheet to emit light in proportion to the stored radiation energy. The light emitted by the stimulable phosphor sheet upon stimulation thereof is photoelectrically detected and converted to electric image signals by a photodetector (i.e. radiation image read-out processing is carried out), and the radiation image of the object is reproduced as a visible image by use of the image signals on a recording medium such as a photographic film, a display device such as a cathode ray tube (CRT), or the like.
As disclosed in, for example, U.S. Pat. No. 4,400,619 and Japanese Unexamined Patent Publication No. 56(1981)-12599, the stimulable phosphor sheet can be reused repeatedly for image recording by exposing the stimulable phosphor sheet to light or heat to release radiation energy remaining thereon after radiation image read-out has been carried out.
However, the stimulable phosphor sheet has a very high sensitivity, and therefore stores energy of radiations emitted by radioactive isotopes such as Ra226 and K40, which are contained in a trace amount in the stimulable phosphor of the stimulable phosphor sheet, or energy of environmental radiations such as cosmic rays and radiations emitted by radioactive isotopes contained in paints on indoor walls or the like. In the case where recording and reproduction of a radiation image are carried out by use of the stimulable phosphor sheet storing these types of radiation energy, small black points are caused by the radiation energy to appear in a reproduced visible image. Such black points adversely affect the image quality of the reproduced visible image.
If noise components generating the black points could be detected from the image signals obtained by the aforesaid radiation image read-out processing, it would become possible to prevent generation of the black points in the reproduced visible radiation image by processing the image signals for eliminating the noise components.